This invention relates to circuits for reducing acoustic feedback in electro-acoustic systems and, in particular, to reducing feedback by the use of a low frequency noise signal.
Sound waves are slight variations in air pressure that a microphone converts into an electrical signal of varying amplitude. In a speaker phone or other electro-acoustical device, the electrical signal is amplified and converted back into sound waves by one or more speakers.
In theory, a signal passes through a system once, never to return. In a "speaker phone," there can be a significant level of acoustic coupling between the speakers and the microphone. When the output of an amplifier is coupled to the input of the amplifier, one has feedback, a closed loop with the potential to oscillate. Typically, speaker phones detect feedback and respond by turning off one channel to the conversation. The telephone is reduced to "half duplex" or simplex operation, i.e. single direction at a time. The speaker at one station must stop and the circuits must re-settle before a person at the other station can be heard. Unless people happen to be used to speaking on such a system, amateur radio operators for example, it can be very difficult to carry on a conversation.
There are two difficulties in eliminating feedback in an acoustic system. One difficulty is determining whether the signal passing through the amplifier is from an echo or from an original sound. A second difficulty is determining the travel time of the echo. U.S. Pat. No. 5,412,734 (Thomasson) discloses an system for resolving these difficulties by tagging an original sound with an inaudible replica of that sound, wherein the replica is a frequency modulated (FM) high frequency carrier. U.S. Pat. No. 5,649,019 (Thomasson) discloses a similar system, wherein the replica is a pulse width modulated (PWM) high frequency carrier.
Although FM and PWM are effective techniques, it is desired to improve the efficiency and effectiveness of echo cancellation using an inaudible acoustic tag. Above 20 kHz, the characteristics of acoustic transducers and of a room change significantly with frequency. Also, using a signal of 20-40 kHz can introduce distortions due to the change in characteristics of the transducers between 20 kHz and 40 kHz. In addition, ultrasonic sounds tend to be highly directional and reflections can cause multiple echoes of nearly the same magnitude. Thus, the ultrasonic sounds my not represent what is happening to the audible sound in a room.
Copending application Ser. No. 09/401,349, filed Sep. 21, 1999, entitled "Echo Cancelling Process with Improved Phase Control" and assigned to the assignee of this application, discloses a system in which coarse and fine delays are made in separate control loops to adjust precisely the delay of an echo for cancellation. Although the particular technique described does not require a tagged signal, it is desired to improve the system by providing a known signal for locking at least the coarse delay control loop.
The time required for a control system to stabilize is called the convergence time. In electronic systems, this time is usually relatively short. In the prior art, many echo cancelling systems perform some sort of test at the beginning of a telephone call and then do not change for the remainder of the call. In an actual telephone call, the characteristics of an echo can change considerably, e.g. a person on a speaker phone moves about a room. Thus, it is desired that the echo cancelling system converge quickly, thereby enabling the system to continuously correct for echo during a call.
During a telephone call, the amplitude of the sound is not uniform, even if averaged. For a control system to converge, it is preferred to have a continuous signal with which to operate. Using an automatic gain control (AGC) is undesirable because the AGC circuit itself is a control loop with its own time for convergence. Tagging with an audible tone would provide a steady signal for the control circuitry but is undesirable in that the tone is perceived by the user and is likely to be annoying, even if intermittent or perhaps especially if intermittent.
In view of the foregoing, it is therefore an object of the invention to provide a low frequency tag for eliminating acoustic echoes.
Another object of the invention is to provide an acoustic tag that is unobtrusive in a telephone system.
A further object of the invention is to provide an acoustic tag that is substantially non-directional.
Another object of the invention is to provide a tag that is easily found and quickly converged upon.
A further object of the invention is to provide a reliable signal upon which the echo cancelling system can converge.